This invention relates to a device for use in the drilling of directional (i.e., non-vertical) boreholes or wells. More particularly, this invention relates to a device for use in navigating and guiding the well being drilled along a particular stratum by detecting the approach to a boundary of the stratum and the direction of said approach. The device of the present invention detects this information which is then used to change the direction of the well away from the boundary of the stratum and back towards the preselected and desired path.
In many applications, it is necessary and desirable to drill a section of a well along a particular stratum. This stratum may be horizontal or inclined and therefore the well which is drilled therethrough must be guided so as not to deviate into adjacent strata. Accordingly, the directional driller must be provided with information which indicates when the drill is approaching a boundary between two adjacent strata, and from which direction the approach is occurring. One particularly well known application of this type of directional drilling is found in coal mining wherein boreholes are desired within a seam of coal which is typically in a horizontal or inclined stratum and is sandwiched between layers of shale or other materials. In this application, it is desired to keep the well or bore hole within the coal seam and therefore the directional driller must know when the well is approaching a shale/coal interface so that the well can be directed back into the coal seam.
One known prior art method of directional drilling within a particular stratum utilizes a sensing and detector device which is based on a gamma radiation sensor. Significantly, unlike other known gamma radiation sensors, the gamma radiation sensor used in directional drilling within a particular stratum utilizes a tungsten or lead shield covering a portion of its circumference. As a result, gamma radiation from one direction is sensed at full strength while radiation from the opposite direction is minimized by the shield. This device makes use of the fact that adjacent strata generate different amounts of radiation. Thus, by rotating the gamma radiation sensor about its axis, a nearby boundary between strata may be detected. The direction of this boundary is determined by rotating the sensor and measuring the angle at which the radiation received is maximum or minimum.
While suitable for its intended purposes, there is a perceived need in the art for additional means for measuring the boundary layer between adjacent strata during such directional drilling. This additional sensing means may be used both separately or in conjunction with exisiting gamma radiation sensor devices. An additional independent sensing means would provide an important backup system to gamma radiation systems and limit the possibility for error.